Terumos 5:6-7

Let’s say that a seah of terumah falls into less than 100 seah of chullin (non-sanctified food), causing it to become medumah, and some of that medumah then fell into other chullin. Rabbi Eliezer says that it renders the second chullin medumah just as definite terumah would. The Sages say that medumah only creates medumah proportionate to the percentage of terumah in the original mixture. Furthermore, when it comes to terumah leaven in chullin dough that falls into other chullin dough, the leaven only affects the second dough proportionate to the leaven in the original mixture. Similarly, drawn water only disqualifies a mikvah based on proportions.

Terumos 5:7

Let’s say that a seah of terumah fell into 100 seah of chullin but it was removed, then another seah of terumah fell in and was removed, and then another seah of terumah fell in, etc. The mixture remains permitted until the amount of terumah exceeds that of the chullin.